(1) Field of the Invention
The present invention relates in general to rotary regenerative air heaters for transferring waste heat from the flue gas exiting from a boiler to the incoming combustion air, and in particular to a new and novel labyrinth articulation joint for use in the seal frame of a rotary regenerative air heater.
(2) Description of the Related Art
In one type of rotary regenerative air heater, a cylindrical heat exchange mass and associated containment structure, called the stator, is positioned stationary between the inlets and outlets of the air and gas ducts. The stator is a radially compartmented steel shell packed with a multiplicity of plates that comprise the heating surface, arranged to provide an axial passage therethrough, such that the gas and air flow in an axial direction through the cylindrical heat exchange mass. The plates embody shapes, materials and thicknesses designed to provide optimum heat transfer, low pressure drop, corrosion resistance and ease of cleaning.
Air ducts at each axial end of the stator include air duct hoods, coaxially aligned with the cylindrical heat exchange mass, which are secured to a central drive shaft for coaxial rotation in relation to the cylindrical heat exchange mass. Each of the air duct hoods comprises a respective central flow inlet or outlet passage centrally mounted between, and in fluid communication with, two diametrically opposite hood segments or sectors for the passage of air to or from the heating mass. The hood segments of each of the air duct hoods are generally pie-shaped and diametrically and circumferentially spaced from each other. The air duct hoods, located at opposite ends of the heat exchange mass, rotate synchronously so that radial sectors of the mass of heat exchange plates and alternately exposed first to the hot flue gas exiting from the boiler and then to the incoming combustion air. The boiler flue gas heats the mass of heat exchange plates, whhich then transfer the heat to the cooler incoming combustion air in a continuous cycle of regenerative heating and cooling. By having two separate hood segments, the heating elements of the stator are thus exposed to two gas and two air cycles per revolution, thereby minimizing the swing in air heater "cold end" temperature during operation.
The gas ducts are arranged in a stationary position at the opposite ends of the stator and surround the rotating air duct hoods. The gas ducts direct the incoming boiler flue gas past the rotating air duct hoods and through those portions of the mass of heat exchange plates in the stator that are not covered by the rotating air duct hoods at any given time.
Since two separate moving fluid streams are arranged to alternately pass over a stationary common area (i.e. the heating surface in the stator), means for preventing intermixing of these streams at other locations must be provided. Prevention of this intermixing of the fluid streams is complicated, however, by the thermally-induced distortion of the stator during operation. Briefly, due to the temperature difference between the "hot end" (gas inlet/air outlet) side and "cold end" (gas outlet/air inlet) side of the air heater during operation, the "hot end" side of the air heater becomes convex, while the "cold end" side of the air heater becomes concave. Any sealing mechanism, then, must be able to accommodate this distortion. Sealing between the stationary stator and the rotating air duct hoods is thus achieved by sealing strips that are attached to articulating seal frames. These articulating seal frames are spring mounted to the rotating air duct hoods, and allow the sealing strips to conform to the curvature of the stator surface and maintain an effective seal between the relatively high pressure "air side" of the air heater and the relatively low pressure "gas side" of the air heater. The articulating seal frames utilize articulating joints located in the radial portions of the seal frames to accommodate the above-described thermally induced stator distortion. An expansion joint is connected between the seal frame and the air duct hoods to partially accommodate the relative thermal displacement of these elements, as well.
The gas and air flow through the rotary regenerative air heater is usually accomplished by fan means. Due to the air heater's position in the air/gas paths of the boiler, the air side of the air heater is generally at a higher operating pressure than the gas side of the air heater, as was indicated above. Since, in a rotary regenerative air heater, the heating surface is alternately exposed to the gas and air streams, separated at any given instant at the sliding interface by the above-described sealing strips, a potential for leakage from the air side to the gas side is always present. This leakage is detrimental to the air heater. The flue gas exiting from a boiler, especially in a coal-fired installation, contains highly erosive particles, most notably silica, that eventually find their way to the air heater. As the hoods rotate over the stator, thereby alternately passing air through heating surface where particle entrained flue gas had been present only moments before, entrainment of some of these particles in the air stream becomes inevitable. Since the air side of the air heater operates at a higher pressure than the gas side, any leakage air containing these particles "sandblasts" or erodes any surface over which it passes. It has been discovered that some currently operating air heaters, especially those at boiler installations firing high silica lignite coal fuels, have been experiencing severe erosion at the very articulating joints that enable the seal frames and attached sealing strips to conform to the curvature of the stator. Analysis of the problem revealed that the "old style" articulating joint, while permitting the proper degree of flexion for sealing, did so by leaving a straight-through, unobstructed gap at the center of the joint, thereby allowing leakage air to pass through and erode the joint. Left unchecked, erosion of these articulating joints can lead to the structural failure of the sealing frame and damage to the stator and sealing strips. Since repair/replacement of a damaged articulating joint requires removing the affected air heater from service, utilities are reluctant to have a unit "down" for any extended period of time. Further, since the problem was occurring at operating air heater installations, any solution to the problem had to fit within the confines of the existing seal frame structure, which was essentially two wide flange structural members connected end-to-end at the articulating joint. A redesign of the entire seal frame was unacceptable in terms of time and cost.
The use of articulating joints, per se, to enhance the sealing between a stator and a rotating surface is known in the art. Penny (U.S. Pat. No. 3,882,927) teaches an articulating seal for a rotary regenerative heat exchanger, (which could be generally described as a "block-channel" design) having a plurality of blocks arranged end-to-end in a guide housing, that form the pivoting articulating joints of the seal. One of the adjacent end faces of the blocks may be of cylindrical concave shape and the adjacent end of the cooperating block may be of complimentary cylindrical convex shape. Other embodiments show both adjacent end faces of the articulating joint to have a concave cylindrical shape, a cylindrical roller being positioned between the blocks and located in the concave end faces, or the use of a knuckle joint design/hinge pin arrangement. Spring means or fluid pressure acting in the space between the blocks and the guide housing act to urge the blocks against the matrix disc of the air heater. Pereira (U.S. Pat. No. 4,185,686) also teaches a sealing apparatus of the above-described "block-channel" design, for use in a rotary regenerative heat exchanger, that also allows for articulation along the length of the seal by means of tongue and groove joints for inter-engagement. Guillot (U.S. Pat. No. 4,024,905) and Handa (U.S. Pat. No. 4,084,634) also teach block-channel seal configurations; Guillot using U-shaped compression members disposed within U-shaped shoes while Handa uses rectangularly shaped friction shoes that are biased into sliding contact with one of the walls of the groove by a springy plate. Both the articulating members themselves and the channels/grooves associated therewith are used to effect sealing. Gignac (U.S. Pat. No. 3,703,297) teaches an articulates link seal assembly comprised of a plurality of modular seal blocks of resilient elastomer material, used to seal one pipe or conduit to another, or to a wall opening or the like. The ends of the seal blocks are similar to those described in Penny, supra, except that two pressure plates (mounted at the sides of the blocks) and two bolts are used to interconnect them. When the seal assembly is wrapped around a first conduit, axial compression of the bolts causes the elastomeric material to expand circumferentially and seal against a second conduit or wall into which the first conduit is inserted. No mention is made, however, of permitting relative movement between the two sealed together elements--in fact, the application of the invention would suggest that such relative movement is undesirable.
It has thus become desirable to develop an improved articulation joint for use in the seal frame of a rotary regenerative air heater. Such an articulation joint must easily replace the "old style" articulating joint, provide for the same degree of flexion of the seal frame in which it would be installed, and yet reduce/eliminate the erosive leakage problem at the joint.